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Henan traffic science and technology information > > Construction management of dynamic compaction method for treating liquefied foundation
Highway engineering geological information
(A) the construction management of dynamic compaction method to deal with liquefied foundation
In northern Jiangsu (such as Xuzhou and Suqian). ), the flooded sediments of the waste Yellow River are widely distributed, mainly clayey silt and loam-fine sand, with shallow burial, high groundwater level, low bearing capacity of natural foundation and easy liquefaction under earthquake action. Liquefaction of foundation is the main form of structural damage, and at the same time, this area is affected by the main earthquake danger zone in our province-the Tan-Lu earthquake zone. Therefore, in the construction of expressways such as Jingfu, Xu Su, Xu Lian, Ningsuxu and Huaiyi, which are the main national highways in this area, large-scale liquefaction foundation treatment problems will inevitably be encountered. According to the Code for Seismic Design of Highway Engineering (JTJ 004-89), the expressway must be treated with liquefied foundation, which is the fundamental measure to reduce the earthquake disaster. Therefore, how to control and manage the construction of liquefied foundation to make it economical, effective, safe and reliable is of great practical significance to ensure the normal operation of expressway after completion and reduce earthquake disasters.
General situation of research on 1 liquefied foundation at home and abroad
The analysis and treatment of foundation liquefaction has always been one of the main research topics of soil dynamics. Hazen discovered liquefaction for the first time in the "dynamic fill dam". A in 1920 is used to describe the destruction of the filling dam of Caravelas. In 1936, casagrande first gave a method to distinguish sand liquefaction-critical void ratio method. In 1950s, scholars all over the world made extensive research on sand liquefaction, including: the mechanism of sand liquefaction, the prediction method of sand liquefaction, and the foundation treatment of sand liquefaction.
Liquefaction refers to the process that granular materials (sand, silt and even gravel) change from solid to liquid due to the increase of pore water pressure and the decrease of effective stress. The factors affecting liquefaction are: ① particle gradation, including clay and silt content, and average particle size d50；; ② Water permeability; ③ Relative density; ④ structure; ⑤ saturation; ⑥ Dynamic load, including amplitude, duration, etc.
China's Code for Seismic Design of Industrial and Civil Buildings (TJ11-78+0-78) gives the sand liquefaction discrimination formula based on the data of 8 major earthquakes before 197 1 and referring to the relevant research results of the United States and Japan. The current code "Code for Seismic Design of Buildings" (GBJ 1 1-89), based on a systematic study of the Haicheng and Tangshan earthquakes, combined with a large number of foreign data, revised the original code, adopted the two-step evaluation principle, and revised the formula of critical SPT blow count to make it more realistic. This is supplemented in the national standard "Code for Geotechnical Engineering Investigation" (GB 5002 1-94), and the formulas of critical value of liquefaction specific penetration resistance and critical value of liquefaction shear wave velocity are given for liquefaction discrimination. The above two-step evaluation principle is basically followed in highway engineering, and the critical SPT blow count discrimination method is adopted. According to the research results of highway engineering, the calculation formula of critical SPT blow count is given. These specifications have been widely used in China's engineering field.
2. Determination of liquefaction foundation treatment scheme of high-grade highway.
Whether the liquefied foundation treatment is proper or not is related to the quality, investment and progress of the whole project. Therefore, its importance is increasingly recognized by people. For the treatment of liquefied soil in such a large area as expressway, dynamic compaction method and dry vibration gravel pile method are the first choice. When the section of fully liquefied foundation is long, the area to be treated is large, and there are no villages and important structures near the outer edge of highway, dynamic compaction is an ideal foundation treatment method.
Dynamic compaction was first established by Menard Technology Company in the late 1960s. This method makes the weight of 80…400kN fall freely from the falling distance of 6-40m, giving the foundation impact and vibration, thus improving the strength of the foundation soil and reducing its compressibility. Dynamic compaction is often used to reinforce various foundation soils, such as gravel, sandy soil, cohesive soil, miscellaneous fill and wet loess. Due to the advantages of simple equipment, fast construction speed, wide application range, material saving, economic feasibility and remarkable effect, after more than 20 years of application and development, dynamic compaction foundation treatment has attracted the attention of engineering circles in various countries, and has been rapidly popularized, achieving great economic and social benefits.
Because the object of dynamic compaction (foundation soil) is very complex, it is generally believed that it is impossible to establish a universal theory applicable to all kinds of foundation soil, but there are still laws to follow for several types of soil that are often encountered in foundation treatment. Practice has proved that the dynamic compaction method must correctly select the dynamic compaction parameters according to the geological conditions and engineering requirements. Usually, the following dynamic compaction parameters are determined by experiments:
(1) Effective reinforcement depth: Effective reinforcement depth is not only an important basis for selecting foundation treatment methods, but also reflects the treatment effect.
(2) Ramming energy: Ramming energy is equal to hammer weight × drop distance.
(3) Optimal tamping energy: Theoretically speaking, under the optimal tamping energy, the pore water pressure in the foundation soil reaches the self-weight pressure of the soil, and such tamping energy is called optimal tamping energy. Therefore, the optimum tamping energy can be determined according to the superposition value of pore water pressure. In sandy soil, the process of pore water pressure growth and dissipation is only a few minutes, so the pore water pressure cannot be superimposed with the increase of tamping energy, and the optimal tamping energy can be determined according to the relationship between the maximum pore water pressure increment and tamping times.
The tamping times of tamping points can be determined according to the relationship curve between tamping times and tamping settlement obtained from field trial tamping, and the following conditions should be met: (1) The ground around the tamping pit should not rise excessively; (2) Because the rammer pit is too deep, it is not difficult to hammer; (3) The settlement of each rammer should not be too small, and it should not be reinforced if it is too small. The tamping times can also be determined by referring to the uplift of the soil around the tamping pit, that is, the tamping times when the vertical compression of the tamping pit is maximum and the surrounding soil uplift is minimum. For saturated fine-grained soil, the number of blows can be determined according to the increase and dissipation of pore water pressure. The number of blows when the reinforced soil layer will liquefy is the number of blows, and the number of blows in the future can also be determined accordingly.
(4) Ramming times: Ramming times shall be determined according to the properties of foundation soil. The foundation soil has low permeability coefficient and high water content, so it needs to be rammed for 3-4 times, otherwise it can be rammed for 2 times, and finally it is rammed with low energy, with the purpose of tamping loose surface soil.
(5) Intermittent time: The so-called intermittent time refers to the time interval between two adjacent tamping. Menard pointed out that once the pore water pressure dissipates, a new compaction operation can be carried out.
(6) Arrangement and spacing of tamping points: In order to make the foundation more uniform after tamping, tamping points can generally be arranged in an equilateral triangle or regular main shape for large-area dynamic compaction, which is more regular and convenient for dynamic compaction construction. Due to the stress diffusion of foundation, the scope of dynamic compaction should be larger than that of foundation, and its specific amplification range can be determined according to the type and importance of structures.
The spacing of tamping points can be determined according to the nature of the foundation soil to be reinforced and the required treatment depth. When the soil quality is poor and the soft soil layer is thick, the spacing of tamping points should be appropriately increased, and when the soft soil layer is thin and there is sand interlayer or rock-filled soil, the spacing of tamping points can be appropriately reduced. If the tamping distance is too small, the reinforcement of adjacent tamping points will form a hard layer in the shallow place, which will affect the transfer of tamping energy to the deep place.
3 quality control and management of dynamic compaction method for liquefied foundation treatment
3. 1 Selection of construction unit
For the dynamic compaction construction unit involved in the construction, the winning bidder of each construction bid section shall first review its construction qualification, reputation and performance, and attach the written evaluation report of the original owner to the unit; No unit may subcontract the dynamic compaction construction to individuals. Each successful bidder shall form a written recommendation report on the qualified construction team after preliminary screening, which shall be submitted to the competent department after being audited by the resident supervisor, and can only enter the site after approval. Do not subcontract or subcontract after entering the site, otherwise, the resident supervision engineer will order the subcontractor to leave immediately at his own risk.
3.2 Construction preparation
The construction organization design shall be reviewed by the resident supervision team, and the supervision team shall put forward written review opinions, which shall be submitted to the representative of the director for approval before construction.
3.3 Construction management
(1) The construction unit shall prepare the tamping point number diagram according to the design requirements, and the number diagram shall be clear, standardized and scientific.
(2) The construction unit must formulate strict safety management measures, and the on-site operators must wear safety helmets, and conduct regular safety inspections on construction machinery. Eye-catching danger warning signs and safety management measures should be set around the dynamic compaction area to prohibit pedestrians and non-construction vehicles from entering the dynamic compaction area to ensure the safety of operators, passers-by and vehicles.
(3) The construction unit shall number the dynamic compactors, and each dynamic compactor must hold a construction permit issued by the supervision unit before dynamic compaction can be carried out.
(4) In addition to hanging the construction permit on the dynamic compaction machine, the construction unit should also hang two eye-catching signs, namely the main mechanical operators and construction technical parameters, and the main mechanical operators must be listed for posts.
(5) The construction unit shall formulate the construction points for the field personnel to implement.
(6) Before laying the cushion, the original ground should be cleared and leveled, and a section should be made every 20 meters, and five measuring points should be designated for each section to measure the elevation after clearing the platform.
(7) Measure the elevation of the corresponding measuring points before and after the cushion is laid with a level, and preliminarily determine the thickness of the cushion. Every 20 meters, specify five measuring points on each section, and then dig a pit with a depth of 1 on each section to further determine the cushion thickness (pit control must be at the measuring point position).
(8) The width of cushion shall be measured with steel rule every 20 meters.
(9) Arrange tamping points according to design requirements, and measure the location and arrangement of tamping points with a steel ruler at the frequency of 100%.
(10) The rammer must be weighed. Before dynamic compaction construction, the tamping energy must be tested and meet the design requirements. Every compaction 100 times, measure the falling distance of the rammer with a steel ruler.
(1 1) The construction unit must discharge the water accumulated in the rammed pit in time.
(12) The clearance time of main rammer, auxiliary rammer and full rammer should be adjusted according to the site conditions, but the clearance time must be 72 hours. The gap time to be adjusted shall be determined by the site supervision engineer.
(13) When encountering a high-voltage power line that does not need to be dismantled, the construction unit must arrange a centralized construction plan, and the city's top management applies to the power supply department for a temporary power outage.
(14) The construction personnel shall carefully record the dynamic compaction construction, and the records shall be clear and true.
(15) Construction personnel must pay attention to the treated road sections, and report to the resident supervision team and relevant departments in time if any abnormal situation is found.
(16) The structure in the dynamic compaction area can not be constructed under the structure until the dynamic compaction is completed.
4. Quality inspection and evaluation of sand liquefaction foundation treated by dynamic compaction method
4. 1 Basic requirements
The specifications and quality of the crushed stones of the crushed stone cushion must meet the design requirements. Dynamic compaction construction must be carried out according to the technical parameters determined by the tamping point. Take the number of tamping points as the construction control value.
4.2 Table of Measurement Items
Table 1 Testing Items of Dynamic Compaction Method for Treating Sandy Liquefied Foundation
Project order
cheque
Project specified value
Or allowable deviation
Inspection method and frequency
Prescribed score
1
impact energy
No less than design.
1 time/worksite inspection construction records
15
2
Tamping times
Conform to design
Check the construction records
15
three
Cushion thickness
No less than design.
4 positions/200m
15
four
Cushion width
No less than design.
4 positions/200m
15
five
SPT blow count
Conform to design
2 locations/sites
20
six
Rayleigh wave
≥ 200m/s
1/ site
20
Note: (1) standard penetration test, 3 points /5000? And not less than 3 o'clock. Random hole position
Layout. Special areas are properly encrypted.
(2) The Rayleigh wave method (SSW) takes 1 point /40m as the benchmark, and sets the intersection points at 15m on both sides of the central line.
4.3 Appearance appraisal
(1) Before filling the gravel cushion, the surface must be cleaned and leveled, and there are no obvious convex and concave points. Leveling does not meet the requirements, 2 points.
(2) The accumulated water in the rammer pit should be eliminated in time, and 2 points will be deducted if it does not meet the requirements.
(3) After compaction, the site is flat and there is no local uplift. If it does not meet the requirements, 2 points will be deducted.
4.4 subentry project quality grade evaluation
(1) The scores of 85 and above are excellent; 70~85 points qualified; Those below 70 points are unqualified.
(2) If the SPT blow count and Rayleigh wave are unqualified, the sub-project is unqualified, and it can be reinforced to re-evaluate its quality grade.
5 conclusion
In the practice of dynamic compaction construction of liquefied foundation in Beijing-Fuzhou, Xu Su, Ningsuxu and other expressways, due to the great attention of the construction, supervision and construction units, the dynamic compaction method is strictly controlled in accordance with the relevant quality requirements and "Construction Guidance Opinions", especially after repeated tests and demonstrations, the appropriate parameters are selected to guide the construction, so that the dynamic compaction method can meet the design requirements. The dynamic consolidation of liquefied foundation in the treatment section can be seen from the post-construction subgrade settlement observation of the completed expressway treatment section.
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